The present invention relates to improvements in a liquid crystal apparatus.
Recently, liquid crystal apparatuses are applied in image display apparatuses such as television receivers and computer displays, computer terminals, and electrophotographic printers used as office equipment or the like.
A liquid crystal apparatus used in an image display or an electrophotographic printer comprises a plurality of light shutters (microshutters) which are densely arranged in order to control a light transmission amount. The light shutters comprise a liquid crystal composition layer, a pair of opposing substrates sandwiching the liquid crystal layer therebetween, and opposing electrodes respectively formed on the substrates. Such a liquid crystal apparatus is described in, e.g., U.S. Pat. No. 4,386,836.
The liquid crystal apparatus is generally driven in such a manner that voltages higher or lower than a threshold voltage of an electro-optic effect are selectively applied to the shutters. However, in this driving method, operation time of the apparatus is delayed. For example, in the case of using a liquid crystal composition having a positive dielectric anisotropy, when a voltage higher than a threshold voltage is applied, liquid crystal molecules are operated immediately. However, when a voltage lower than the threshold voltage is applied and the liquid crystal molecules are aligned parallel to the substrate, the liquid crystal molecules have poor response time. This is because the liquid crystal molecules are operated only by an alignment regulating force acting between the liquid crystal composition and an alignment film on the substrate.
For this reason, when the light shutters must be driven at high speed so as to obtain high contrast, a so-called two frequency addressing scheme is adopted.
According to the two frequency addressing scheme, when a high frequency electric field is applied to the liquid crystal composition, the liquid crystal molecules are aligned to be perpendicular to the applied electric field. When a low frequency electric field is applied, the liquid crystal molecules are aligned to be parallel to the electric field. In this addressing scheme, the electric field is applied to the liquid crystal composition whether the liquid crystal molecules are to be aligned parallel or perpendicular to the substrates. For this reason, the liquid crystal molecules can always be operated at high speed. In a liquid crystal apparatus of positive-display TN type (twisted nematic type: polarization axes of polarizing plates are perpendicular to each other) driven according to this scheme, when a high frequency voltage is applied, shutters are opened (light can be transmitted), and when a low frequency voltage is applied, the shutters are closed (light cannot be transmitted). In a liquid crystal of G-H type (guest-host effect type), when a high frequency voltage is applied, shutters are closed (light cannot be transmitted, i.e., the shutters are colored), and when a low frequency voltage is applied, the shutters are opened (light can be transmitted, i.e., the shutters are not colored).
When the liquid crystal apparatus is driven by the two frequency addressing scheme, high frequency (100 kHz or higher) and low frequency electric fields are applied. In this case, the response time of the liquid crystal composition is rendered poor when a low frequency electric field is applied after a high frequency electric field has been applied to the liquid crystal composition for a long period of time. This drawback is caused by a hysteresis effect (high frequency hysteresis effect) of the liquid crystal composition. The hysteresis effect of the liquid crystal composition is notable particularly when a high frequency voltage is applied. For this reason, if a high frequency voltage is applied for a long period of time, its influence remains in the liquid crystal composition considerably. Thus, when a low frequency voltage is applied, the liquid crystal composition does not respond quickly. In other words, when a high or low frequency electric field is applied, the liquid crystal molecules are aligned to be parallel or perpendicular to the plane of substrates. In this case, the liquid crystal molecules are not completely aligned parallel or perpendicular to the plane of substrates but are operated within a given range. However, when the high frequency electric field is applied for a long period of time, the liquid crystal molecules are almost completely aligned parallel to the plane of substrates. Therefore, when the high frequency electric field is applied for a long period of time, and the low frequency electric field is applied thereafter, the liquid crystal molecules do not respond quickly.
Influence of the hysteresis effect of a high frequency electric field depends on the strength of such electric field applied to the liquid crystal composition. If the applied electric field is strong, the hysteresis effect due to the high frequency voltage is considerable.
For this reason, when the conventional liquid crystal apparatus is driven by the two frequency addressing scheme, in the case of positive-display TN type, an operation for switching the shutters from an open state to a close state becomes slow. In the G-H type liquid crystal apparatus, an operation for switching shutters from a close state to an open state becomes slow. Therefore, contrast between the shutter closed and open states may be incorrect.
For example, when a conventional liquid crystal apparatus used in an electrophotographic printer is driven by the two frequency addressing scheme, in the case of the TN type, some initially printed pixels become defective or are not printed due to a delay in the closing operation of the shutters. In the G-H type liquid crystal apparatus, some finally printed pixels are left printed due to a delay in the opening operation of the shutters.
This drawback is particularly notable in a liquid crystal apparatus in which the shutters are arranged in a matrix and are time-divisionally driven by the two frequency addressing scheme. This is because, in this case, high frequency components of the electric field applied to the liquid crystal composition increase.
Moreover, in a liquid crystal apparatus using a narrow rectangular liquid crystal shutter portion (a portion in which the liquid crystal composition forming the light shutters is sealed), a width of a cell container in which the liquid crystal composition is sealed is small. For this reason, the cell container cannot deform upon expansion or shrinkage of the liquid crystal composition due to a change in ambient temperature. As a result, in accordance with a change in ambient temperature, the liquid crystal composition in the cell container has high or low pressure. When the composition in the cell container has low pressure, air bubbles can be formed in the shutter portion. When the composition in the cell container has high pressure, the liquid crystal composition can leak. Therefore, the liquid crystal apparatus with the above arrangement has poor reliability with respect to a change in ambient temperature.